1. Field of the Invention
This invention relates to lamps having a boron nitride reflective coating. More particularly, this invention relates to an electric incandescent or arc discharge lamp including a light-transmissive envelope enclosing a filament or arc within wherein a boron nitride coating is disposed on a portion of the envelope surface.
2. Background of the Disclosure
Coatings which reflect both visible light radiation and infrared radiation have been used on various types of lamps for years. White, opaque, heat-resistant coatings containing refractory metal oxide such as zirconia, alumina, titania, etc., with a glass frit binder or a binder such as a mixture of boric acid and silica, have been used on the ends of arc tubes of high intensity discharge lamps, such as metal halide arc discharge lamps, to prevent ionized metal halide in the arc tube from condensing on the relatively cool ends of the arc tube. Similar coatings have been used on incandescent lamps, such as linear quartz heat lamps, wherein the coating is applied to a portion of the outer surface of the fused quartz lamp envelope as an integral reflector for reflecting both the visible and infrared radiation emitted by the filament. Using an integral reflective coating on half of the linear surface of the envelope of a heat lamp maximizes the radiant energy emitted in the direction of the object to be heated, while minimizing the radiant energy emitted from the opposite side. This enables one to use a smaller fixture in which the heat lamp is mounted, because less heat reflecting surface is needed in the fixture. Such fixtures usually have a polished metal or other reflecting surface which reflects heat emitted by the lamp in the intended direction. Multilayer, thin film light interference coatings have also been employed in various types of lamps as coatings for reflecting various portions of the spectrum, including infrared. All of these coatings are expensive, costly and difficult to apply. Hence there is a need for a heat resistant reflective coating that is relatively inexpensive, easy to apply and which will stand up under the high temperature conditions and thermal cycling that such coatings experience on incandescent and arc discharge lamps.